THE ASTROPHYSICAL JOURNAL, 535 : 991È999, 2000 June 1
( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
CARBON ISOTOPE ABUNDANCES IN COMETS
SUSAN WYCKOFF,1 MARVIN KLEINE,2 BRUCE A. PETERSON,3 PETER A. WEHINGER4, AND LUCY M. ZIURYS4
wycko†=asu.edu, marvin.l.kleine=lmco.com, peterson=anu.mssso.edu.au, pwehinger=as.arizona.edu, ziurys=as.arizona.edu
Received 1999 June 4 ; accepted 2000 January 19
ABSTRACT
Rotational lines of 13C14N have been identiÐed in high-resolution (j/*j D 60,000) echelle spectra of
the CN B2&`ÈX2&` (0È0) band in three comets. The 12C/13C abundance ratios determined using a full
Ñuorescence excitation model for comets Levy (C/1990 K1), Austin (C/1989 X1), and Okazaki-LevyRudenko (C/1989 XIX) are 90 ^ 10, 85 ^ 20, and 93 ^ 20, respectively, consistent with the solar system
ratio, 90. A lower limit for the nitrogen isotope ratio, 12C14N/12C15N Z 200, found for comet Levy is
consistent with previous comet measurements and the solar system value, 272. The mean CN carbon
isotope ratio in the Ðve comets measured to date is 12C14N/12C15N \ 90 ^ 10, and the mean for the
three molecular species C , HCN, and CN measured in nine comets is 101 ^ 15. Consistency of the
2
cometary carbon isotope ratios with the bulk solar system value indicates (1) chemical homogeneity in
the outer protosolar nebula, (2) minimal isotopic fractionation in the protosolar precursor molecular
cloud, and (3) that comets formed coevally with the solar system. The 14% di†erence between the solar
system (90) and the present solar neighborhood interstellar 12C/13C ratio (77 ^ 7) may be indicative of
signiÐcant Galactic 13C enrichment over the past 4.6 Gyr. However, even though models can match to
within a factor of 2 the solar system abundances, including the carbon isotope ratio, other evidence suggests that simple homogeneous Galactic evolution models may not be adequate to explain detailed
stellar and interstellar abundances in the Galaxy.
Subject headings : comets : general È ISM : abundances È solar system : formation
1.
INTRODUCTION
and hydrostatic helium burning in massive, intermediateand low-mass stars. Carbon enrichment of the Galactic
ISM has occurred sporadically by supernovae and continuously by red giant stellar winds. Galactic chemical evolution models that match (within a factor of 2) the solar
system elemental and isotopic abundances from hydrogen
to zinc have been constructed (e.g., Timmes, Woosley, &
Weaver 1995). One chemical evolution model consistent
with the observed solar system abundances indicates that
the bulk of the solar system carbon (about two-thirds) was
generated by Type II and Type Ia supernovae, and the
remaining one-third of the solar system carbon can be
attributed to low- and intermediate-mass (M [ 8 M )
_
asymptotic branch stars that have dredged up 13C-enriched
material and gradually expelled it into the ISM (e.g., Iben &
Truran 1978 ; Timmes et al. 1995).
Although the bulk volatile carbon isotope abundance
ratios measured in solar system objects agree with the solar
value to within about 25%, at microscopic scales large
ranges in the carbon isotope ratios have been found among
individual small grains in the coma of comet Halley
(Jessberger & Kissel 1991), 12C/13C D 1È5000, and in primitive meteorites, 12C/13C D 2È7000 (Amari et al. 1993 ;
Anders & Zinner 1993, 1994). The huge ranges in the
12C/13C ratios indicate that these microscopic particles are
preserved circumstellar grains that have survived both
interstellar and early solar system chemical and physical
processing. The presolar grains comprise an insigniÐcant
fraction (\1%) of the bulk mass of the primitive meteorites
(Anders & Zinner 1994) but may be representative of cometary dust particles, as indicated by analysis of a small
sample of Halley particles (Jessberger & Kissel 1991 ; Eberhardt 2000).
The carbon isotope ratios presented here have been
determined from CN coma emission spectra of three
comets. In ° 2 we describe the observations. The Ñuorescence excitation model is discussed in ° 3. In ° 4 we compare
Carbon isotope abundance ratios in comets provide constraints on the origin of comets and conditions in the outer
protosolar nebula. Recent analyses of carbon isotope ratios
in comets indicate agreement with the solar system value for
CN in comet Halley (Jaworski & Tatum 1991 ; Kleine et al.
1995) and for HCN in comet Hale-Bopp (Jewitt et al. 1997 ;
Ziurys et al. 1999). The bulk carbon isotope abundance
ratio in the solar system determined for the sun, the outer
planets, and comets is 12C/13C \ 90 (Anders & Grevesse
1989 ; Jaworski & Tatum 1991 ; Kleine et al. 1995 ; Wiedeman, Bjoraker, & Jennings 1991 ; Jewitt et al. 1997). This
ratio represents the carbon isotope abundance present in
the protosolar nebula 4.6 Gyr ago when the solar system
formed, since negligible isotopic fractionation has occurred
since that time (Lecluse et al. 1998).
The carbon isotope ratio measured in the present-day
interstellar medium (ISM) toward the Galactic center is
about 20, increasing to 53 ^ 4 at the 4 kpc molecular ring
and to 77 ^ 7 at the solar system ring (8.5 kpc ; Wilson &
Rood 1994). Consistency in the carbon isotope abundance
ratios determined from several molecular species (CH`,
CO, CN, and HCN) indicates that these ISM carbon
isotope ratios are relatively well determined and free from
chemical fractionation e†ects (Crane, Hegyi, & Lambert
1991 ; Hawkins, Craig, & Meyer 1993 ; Wilson & Rood
1994).
The two stable isotopes of carbon, 12C and 13C, have
been produced over the history of the Galaxy by explosive
1 Department of Physics and Astronomy, Arizona State University,
Tempe, AZ 85287.
2 Lockheed-Martin Corporation, P.O. Box 85, LitchÐeld Park, AZ
85340.
3 Research School of Astronomy and Astrophysics, Institute of
Advanced Studies, Australian National University, Private Bag, Weston
Creek ACT 2611, Australia.
4 Steward Observatory, University of Arizona, Tucson, AZ 85721.
991
992
WYCKOFF ET AL.
Vol. 535
the Ñuorescence model with the observed spectra. In ° 5 we
discuss the isotope abundance analysis of the 12C/13C lines,
and in ° 6 we compare and discuss our results with those of
previous analyses. Our conclusions are summarized in ° 6.1.
2.
OBSERVATIONS
Fluorescence spectra of resolved rotational lines in the
R-branch of the (0È0) band of the CN B2&`ÈX2&` system
were acquired for three bright comets using the 1.9 m telescope located at Mount Stromlo Observatory. The observations collected over a period of 2 yr utilized the same
experimental setup in each instance. The detector was a
photon-counting array, consisting of two microchannel
plate intensiÐers coupled to a Fairchild CCD attached to a
coude echelle spectrograph (Stapinski, Rodgers, & Ellis
1981). The measured spectral line FWHM intensity for the
three comets, C/Austin, C/Levy, and C/Okazaki-LevyRudenko (OLR) were 61, 60, and 52 mAŽ , respectively. The
spectral resolution, j/*j, was 60,000, with a spectral sampling of approximately 20 mAŽ . In Table 1 we summarize the
observations for each comet, where r is the heliocentric
h
distance, r5 the heliocentric radial velocity,
* the EarthÈ
h
comet distance,
and l ] w the length and width of the
spectrograph slit projected at the comet distances.
The echelle spectra were obtained with the spectrograph
slit centered on each cometÏs center of brightness (i.e., center
of coma). Calibration quartz lamp spectra were taken to
remove the pixel-to-pixel sensitivity variations across the
detector. A thorium-argon arc spectrum was used for preliminary wavelength calibrations and for deÐning the pointspread function (PSF) for the spectra. The cometary
wavelength calibration was performed using the 12C14N
(hereafter CN) lines as described previously (Kleine et al.
1995). CCD dark frames were also taken to characterize the
thermal noise generated at the front end of the photoncounting array. Spectra of the scattered solar continuum in
the twilight sky were used as sky Ñats to remove the system
response imposed on the comet spectra by the telescopedetector system. Further details of the data reduction
unique to this analysis are discussed in Kleine et al. (1995).
The long-slit spectra were collapsed along the spatial
direction to create a one-dimensional spectrum that maximized the signal-to-noise ratios. The collapsed onedimensional spectra extracted from the two-dimensional
spectra for each comet are shown in Figures 1È3, where the
scattered solar continuum reÑected from the dust particles
within the cometary coma has been subtracted from each
spectrum. Weak lines of the P-branch of the 12C14N (1È1)
band are visible at wavelengths shorter than the P-branch
band head located at 3872.5 AŽ . As can be seen in Figures
1È3, the intensity of the P (1È1) band is a fraction of the R
(0È0) band intensity. The underabundant isotopic lines are
weaker than the P-branch lines.
FIG. 1.ÈSpectrum of the R-branch of the CN B2&`ÈX2&` (0È0) band
in comet Austin. Selected lines are identiÐed by the rotational quantum
number of the lower state, R (NA).
FIG. 2.ÈSame as Fig. 1 but for comet Levy
FIG. 3.ÈSame as Fig. 1 but for comet Okazaki-Levy-Rudenko
TABLE 1
SUMMARY OF OBSERVATIONS
Comet
UT Date
r
h
(AU)
r5
h
(km s~1)
*
(AU)
Austin 1989 X1 . . . . . . . . . . . . . . . . . . . . . . . . . . .
Levy 1990 K1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1990 May 26
1990 Sep 10
1990 Sep 11
1989 Dec 15
1.18
1.21
1.20
0.95
]32.57
[17.97
[18.17
]24.54
0.24
0.65
0.67
0.68
Okazaki-Levy-Rudenko 1989 XIX . . . . . .
l ] w (km ] km)
5340
14200
14400
14800
]
]
]
]
210
570
580
590
No. 2, 2000
CARBON ISOTOPE ABUNDANCES IN COMETS
The coincidence between the excitation wavelengths of
the CN molecule and the presence (or absence) of Fraunhofer absorption lines at the corresponding wavelengths in
the solar spectrum governs the CN transition rate from the
ground state to the excited electronic states. Swings (1941)
showed that the changing heliocentric radial velocity of a
comet caused the excitation wavelength to be Doppler
shifted in or out of the Fraunhofer absorption lines in the
solar spectrum, giving rise to continuously changing intensities of the emission lines. The di†erences in the observed
line intensity structure in the R lines of the (0È0) CN band
evident in Figures 1È3 are a direct consequence of variations in the solar Ñuorescing Ñux, the so-called Swings e†ect
(Swings 1941). Figures 1È3 and the Ñuorescence model discussed below demonstrate that Ñuorescence is the dominant
excitation mechanism for CN in comets. An accurate Ñuorescence model of the CN molecule is needed to calculate
the Ñuorescence efficiencies for each CN rotational line as a
function of the cometÏs heliocentric velocity in order to
determine the carbon isotope abundance ratio.
3.
TABLE 2
ROTATIONAL LINE g-FACTOR RATIOS, g(13C14N)/g(12C14N), AT
TIMES OF OBSERVATION
NA
C/Levya
C/Levyb
C/Austin
C/OLR
0 ..........
1 ..........
2 ..........
3 ..........
4 ..........
5 ..........
6 ..........
7 ..........
8 ..........
9 ..........
10 . . . . . . . . .
11 . . . . . . . . .
12 . . . . . . . . .
13 . . . . . . . . .
14 . . . . . . . . .
15 . . . . . . . . .
0.73
0.84
1.12
1.39
0.51
0.98
0.84
1.04
1.17
0.69
1.05
1.29
1.07
1.41
1.09
1.82
0.76
0.83
1.10
1.34
0.58
0.96
0.82
1.02
1.21
0.71
1.03
1.30
1.02
1.39
1.17
2.06
1.85
1.75
1.15
0.66
1.32
0.77
0.62
1.00
1.01
0.60
1.03
0.71
1.08
0.60
1.18
1.09
0.88
1.08
1.34
1.37
2.42
0.77
1.13
2.05
1.07
1.59
1.31
1.35
1.72
0.71
0.67
0.23
a 1990 September 10 observation.
b 1990 September 11 observation.
FLUORESCENCE EXCITATION
Because of the Swings e†ect, the intensity of the individual rotational emission lines must be modeled for each CN
isotope at the precise time each spectrum is observed. Fluorescence efficiencies as a function of heliocentric velocity
were calculated for all stable CN isotopes for each electronic transition within the R-branch of the B2&`ÈX2&`
(0È0) band using our full Ñuorescence model (Kleine et al.
1994). The model accounts for radiative transitions in the
three lowest electronic states, X2&`, A2%, and i2&` and
includes collisions of CN with H O molecules as discussed
2
previously (Kleine et al. 1994, 1995).
For the underabundant isotopes of CN, namely, 13C14N
and 12C15N, the transition wavelength of a given rotational
line is slightly di†erent from that of the normal isotope
because of the dependence of the molecular parameters on
the individual isotopic masses. Therefore, the Ñuorescence
efficiencies for transitions of each of the CN isotopes are
slightly di†erent for each rotational line and vary as a function of time because of the continuous change in the cometÏs
heliocentric radial velocity. The intensity of the incident
solar radiation is determined by the e†ective excitation
wavelength, j , given by
eff
r5
(1)
j \j 1[ h ,
eff
0
c
A B
where j is the vacuum rest wavelength for a given radiative
0 r5 is the heliocentric radial velocity of the CN
transition,
h the velocity of light. The line Ñuorescence
gas, and c is
efficiencies (or g-factors when expressed in units of
photons~1 s~1 molecule~1) computed at the e†ective Ñuorescence wavelengths are used to calculate the isotopic
abundance ratio, N/Ni for an optically thin coma from
N giI
\
,
Ni gIi
993
(2)
where the superscript i refers to the heavier isotopic species
of CN, g is the computed line g-factor, and I is the measured
photon intensity of a given rotational line. The g-factor
ratios, g(13C14N)/g(12C14N) and g(12C15N)/g(12C14N), for
each comet at the time observed are tabulated in Tables 2
and 3, respectively. The error estimate associated with these
ratios is less than 5%È10% for each g-factor. In Tables 2
and 3 NA is the rotational quantum number of the lower
state.
4.
COMPARISON WITH OBSERVATIONS
Synthetic spectra were generated for each comet based on
orbital parameters at the time of observation. The computed line intensities were convolved with an instrument
line proÐle determined from the thorium-argon arc spectrum modiÐed to include Doppler broadening because of
the CN coma expansion velocity. The synthetic Ñuorescence
spectra are compared with the observed spectra in Figures
4È6, where the synthetic spectra have been arbitrarily
shifted 0.25 AŽ to longer wavelengths to facilitate comparison.
The synthetic spectra were computed assuming pure Ñuorescence, ignoring any coma collisions. The CN spectrum of
TABLE 3
ROTATIONAL LINE g-FACTOR RATIOS, g(12C15N)/g(12C14N), AT
TIMES OF OBSERVATION
NA
C/Levya
C/Levyb
C/Austin
C/OLR
0 ..........
1 ..........
2 ..........
3 ..........
4 ..........
5 ..........
6 ..........
7 ..........
8 ..........
9 ..........
10 . . . . . . . . .
11 . . . . . . . . .
12 . . . . . . . . .
13 . . . . . . . . .
14 . . . . . . . . .
15 . . . . . . . . .
1.21
1.11
0.74
0.67
0.30
0.86
0.82
1.25
1.10
1.15
1.13
1.18
1.56
2.47
1.76
2.23
1.21
1.09
0.73
0.65
0.30
0.85
0.82
1.24
1.12
1.18
1.12
1.20
1.52
2.44
1.79
2.23
1.48
1.41
0.64
0.56
1.26
0.73
0.51
1.17
1.13
1.06
2.04
1.23
1.22
0.48
0.73
0.86
0.88
1.08
1.34
1.37
2.42
0.78
1.13
2.05
1.07
1.59
1.31
1.35
1.72
0.71
0.67
0.23
a 1990 September 10 observation.
b 1990 September 11 observation.
994
WYCKOFF ET AL.
Vol. 535
process in cometary coma is well understood and that Ñuorescence excitation clearly dominates collisions and all
other excitation mechanisms in comets.
5.
FIG. 4.ÈComparison of the high-resolution spectrum of B2&`ÈX2&`
(0È0) R-branch (solid line) for comet Austin with a theoretical spectrum
(dotted line) for a pure Ñuorescence calculation. The theoretical spectrum
has been shifted by 0.25 AŽ to facilitate a side-by-side comparison. Selected
rotational lines are labeled with their NA values.
FIG. 5.ÈSame as Fig. 4 but for comet Levy
FIG. 6.ÈSame as Fig. 4 but for comet Okazaki-Levy-Rudenko
the comet C/Levy was shown (Kleine et al. 1995) to exhibit
the e†ects of collisions in the innermost region of the coma.
The e†ects of collisions are minimized in Figure 4 because
of the averaging process due to the collapse of the spectrum
along the spatial direction. The fraction of the Ñuorescing
CN radicals within the observing aperture in comet C/Levy
subject to collisional e†ects is less than 10%. Comparison of
the synthetic CN spectra with the observed cometary
spectra in Figures 4È6 indicates that the Ñuorescence
ISOTOPE ABUNDANCE RATIOS
Many of the isotopic lines are masked by blends with the
stronger emission lines of the 12C14N R (0È0) and P (1È1)
branches. In fact, only a few of the underabundant isotope
emission lines exist in unmasked spectral regions. To
measure the line intensity a 13C14N or 12C15N line must
have a signiÐcant detection level to permit a quantitative
measurement of its Ñux and the line must not be blended
with other lines. Because most of the line g-factor ratios are
close to unity (Tables 2 and 3), the observed line intensities
scale roughly with the isotopic abundances (eq. [2]). For
solar carbon and nitrogen isotopic abundance ratios, 12C/
13C \ 90 and 12N/13N \ 272 (Anders & Grevesse 1989),
the detection and quantitative evaluation of rotational lines
belonging to the underabundant isotopes are primarily
limited by the low signal-to-noise ratios (S/Ns) of the
spectra, as well as line blends. The transition wavelengths
were determined from laboratory spectra for 12C14N
(Prasad et al. 1992) and for 13C14N and 12C15N (Kleine et
al. 1995).
The carbon isotope abundance ratios were determined by
applying an iterative procedure to isolate the spectral signature of the isotopic lines (Kleine et al. 1995). To constitute
an identiÐcation, each underabundant CN isotopic line was
required to have (1) a signiÐcant S/N, (2) a central wavelength corresponding to the laboratory position of a
13C14N or a 12C15N line, (3) an FWHM consistent with the
corresponding 12C14N line, and (4) a symmetric line proÐle.
While the errors associated in measuring the wavelengths
for the strong 12C14N lines are quite small, approximately 3
mAŽ , the accuracies of the measured line positions for the
weak isotopic features are subject to larger errors because of
their smaller signal-to-noise ratios. Therefore, in the
analysis to determine the underabundant isotopic line
Ñuxes, we used computed line positions.
Each CN rotational line is composed of three Ðnestructure components resulting from the unpaired electron
spin coupling with the rotational angular momentum. The
relative spacing among the line triplet increases as a function of the rotational quantum number N. However, the
resolution of the spectrograph is too low to resolve these
individual Ðne-structure components even in the 12C14N
lines. The widths of all CN lines in the observed comet
spectra are therefore a†ected by three factors : (1) the
spectrograph instrument proÐle, (2) the relative spacing of
lines in a triplet, and (3) the expansion velocity of the CN
radicals within the cometary coma. The spectrograph
instrument line proÐle is characterized by a Gaussian core
with broad wings. The e†ective wavelength for each of the
underabundant isotopic rotational lines was computed
using a weighted average of the wavelengths of the two
Ðne-structure components, R and R . Since the strength of
1 each case
2
the third component, RQ , in
is quite small, this
21
component was neglected. The major sources of error
associated with this procedure of identifying and analyzing
the underabundant isotopic lines are primarily due to (1)
the low S/Ns of the weaker isotopic features and (2) blending with lines of the 12C14N band. Once the 13C14N lines
were identiÐed, the isotope abundance ratios were calculated from equation (2) using the measured line intensities
No. 2, 2000
CARBON ISOTOPE ABUNDANCES IN COMETS
extracted in the procedure summarized above and the calculated g-factor ratios listed in Table 2.
5.1. Carbon Isotope Abundance Ratios
In Figures 7È9 we present detailed comparisons of the
observed with the calculated spectra for the three comets
observed, where the intensities of the calculated 13C14N
lines have been scaled by the 12C/13C abundance ratios
derived in the analyses described below. As indicated below,
at the lowest detection levels in our spectra, a number of
weak unidentiÐed lines were also observed in the cometary
spectra. Those weak unidentiÐed features not belonging to
either the 13C14N or the 12C15N bands are labeled with
small letters in Figure 8. Extra CN lines observed in laboratory spectra, but unclassiÐed, are labeled ““ E ÏÏ in the
Ðgures.
5.1.1. C/Austin
After solar continuum subtraction (Fig. 7), the only emission line within the 13C14N band with any signiÐcant Ñux
(S/N of 3.2) is the Ri(8) rotational line (Fig. 7c). The result of
the extraction procedure and model Ðt to determine the Ñux
of this line is shown in Figure 10. The computed value for
the carbon abundance ratio is 85 ^ 20. The error is an estimate based on errors associated with the calculation of the
Ñuorescence efficiency and the measurement procedure used
to isolate and measure the Ñux of the weak isotopic lines.
995
5.1.2. C/L evy
In C/Levy (Fig. 8) three lines, Ri (6), Ri (7), and Ri (8),
belonging to the 13C14N band were identiÐed (Figs. 8c and
8b). A possible detection of the Ri (1) and Ri (2) lines may
also be indicated in the data, but the lines were blended with
the strong 12C14N lines (see Fig. 8d). Only the Ri (7) and Ri
(8) lines were used in our quantitative analysis to determine
the carbon isotope abundance ratio. The Ri (6) line was not
used because of possible contamination with a pseudocontinuum of the P (1È1) lines near this band head and the
possible presence of an extra line (labeled E in Fig. 8b)
found in laboratory CN spectra at the same wavelength
(Kleine et al. 1995). The S/N for the Ri (7) line is 5.7 and for
the Ri (8) line, 4.9. The results of the procedure used to
isolate the isotopic emission lines and the model Ðt to the
observed line are shown in Figures 11 and 12. The measured values for the carbon isotope abundance ratio using
the Ri (7) and Ri (8) lines are 93 ^ 14 and 86 ^ 14, respectively. Since each line represents an independent measurement, the combined result for the 12C14N/13C14N ratio in
C/Levy is 90 ^ 10.
5.1.3. C/Okazaki-L evy-Rudenko
In this comet (Fig. 9) the only 13C14N emission line suitable for quantitative analysis to determine the carbon
isotope ratio was the Ri (8) line (Fig. 9c). The S/N computed
for this feature is 4.5. The calculated Ðt to the observed
FIG. 7a
FIG. 7b
FIG. 7c
FIG. 7d
FIG. 7.ÈComparison of synthetic spectrum (dotted line) with the observed spectrum for comet Austin based on the carbon isotope abundance ratio
derived in this analysis and a solar nitrogen abundance ratio. Positions for the isotopic lines of 12C14N, 13C14N, and the P-branch of the (1È1)12C14N band
are labeled. Positions where extra lines have been identiÐed in laboratory spectra are labeled E. Calculated positions of 13C14N R-branch transitions are
indicated at the bottom of the Ðgure.
996
WYCKOFF ET AL.
Vol. 535
FIG. 8a
FIG. 8b
FIG. 8c
FIG. 8d
FIG. 8.ÈSame as Fig. 7 but for comet Levy
spectrum is shown in Figure 13. Based on this analysis the
carbon isotope abundance ratio derived is 93 ^ 20.
5.2. Nitrogen Isotope Abundance Ratio
Emission lines belonging to the 12C15N band could not
be identiÐed in any of the cometary spectra. Only in the
spectrum of comet Levy (Fig. 8c) were a number of possible
lines belonging to 12C15N observed. These lines are labeled
d, e, and f in Figures 8a and 8c at measured positions,
3868.270, 3869.040, and 3869.808 AŽ , corresponding to the
positions of the 12C15N Ri (11), Ri (10), and Ri (9) lines,
respectively. The di†erences between the observed and
laboratory wavelengths for these lines are 0.0, 0.020, and
0.010 AŽ , respectively. The Ri (9) 12C15N line (predicted to be
at f in Fig. 8) would be the optimal line to observe based on
predicted line intensities. The intensity of the feature labeled
f in Figure 8c was too weak to permit a quantitative
analysis. Instead a 3 p lower limit, 14N/15N [ 200, was
found. This limit is consistent with the solar value (272). The
nitrogen abundance ratio measured from HCN in comet
Hale-Bopp was found to be solar (Jewitt et al. 1997 ; Ziurys
et al. 1999).
The nitrogen abundance ratio computed using line d
(Fig. 8c) measured at 3868.270 AŽ implies a nitrogen abundance ratio 14N/15N \ 79 ^ 20. If this identiÐcation were
correct, then numerous other lines belonging to the 12C15N
band should be easily visible in the observed spectrum.
However, none of the other 12C15N lines was detected at an
intensity consistent with this abundance ratio. Therefore,
we cannot identify line d with 12C15N. The 20 mAŽ error in
the observed position for line e (Fig. 8c) is too large to
attribute to the 12C15N Ri (10) line.
6.
RESULTS AND DISCUSSION
In Table 4 we summarize the carbon isotope ratios for
the three comets investigated together with previously
determined comet carbon isotope ratios (Stawikowski &
TABLE 4
COMET CARBON ISOTOPE ABUNDANCE RATIOS
Comet
12C/13C
Species
Ikeya 1963 Ia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tago-Sato-Kosaka 1969 IXb . . . . . . . . . . . . .
Kohoutek 1973 XIIc . . . . . . . . . . . . . . . . . . . . . .
70 ^ 15
100 ^ 20
115`30
~20
135`60
~45
100`20
~30
89 ^ 17
95 ^ 12
85 ^ 20
90 ^ 10
93 ^ 20
111 ^ 12
110 ^ 15
90 ^ 15
C
2
C
2
C
2
C
2
C
2
CN
CN
CN
CN
CN
HCN
HCN
CN
Kobayashi-Berger-Milon 1975 IXd . . . . . .
Halley 1982 U1e . . . . . . . . . . . . . . . . . . . . . . . . . . .
Austin 1989 X1f . . . . . . . . . . . . . . . . . . . . . . . . . . .
Levy 1990 K1f . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OLR 1989 XIXf . . . . . . . . . . . . . . . . . . . . . . . . . . .
Hale-Bopp 1995 O1g . . . . . . . . . . . . . . . . . . . . . .
a Stawikowski & Greenstein 1964.
b Vanysek 1991.
c Danks et al. 1974.
d Vanysek 1977.
e Jaworski & Tatum 1991 ; Kleine et al. 1995.
f This paper.
g Jewitt et al. 1997 ; Ziurys et al. 1999 ; Lis et al. 1997.
No. 2, 2000
CARBON ISOTOPE ABUNDANCES IN COMETS
997
FIG. 9a
FIG. 9b
FIG. 9c
FIG. 9d
FIG. 9.ÈSame as Fig. 7 but for comet Okazaki-Levy-Rudenko
Greenstein 1964 ; Danks, Lambert, & Arpigny 1974 ;
Vanysek 1977, 1991 ; Jaworski & Tatum 1991 ; Kleine et al.
1995 ; Jewitt et al. 1997 ; Ziurys et al. 1999 ; Lis et al. 1997).
The carbon isotope ratios for the nine comets listed in Table
4 are compared with the solar system and local ISM values
in Figure 14. As Figure 14 indicates, the comet carbon
isotope ratios for all species observed are consistent with
FIG. 10.ÈFinal Ðtted proÐle of the Ri (8) line of 13C14N (dotted line) for
comet Austin is compared to the observed spectrum (solid line) with the
blending of the adjacent P (15) and P (16) lines of the P-branch taken into
account. The S/N of the Ri (8) line is 3.2.
the solar system abundance ratio within the observational
errors. Consistency between the comet and solar system
carbon isotope ratios indicates that this sample of comets
formed coevally with the other bodies in the solar system. If
these comets are typical of all comets, this result indicates
that they originated in the protosolar nebula and were not
captured at later epochs (Clube & Napier 1984, 1985). The
fact that the three carbon-bearing species (Table 4) all give
about the same value for the carbon isotope ratio indicates
that chemical fractionation was not signiÐcant for these
FIG. 11.ÈSame as Fig. 10 but for comet Levy. The S/N of the Ri(8) line
is 4.9.
998
WYCKOFF ET AL.
FIG. 12.ÈFinal Ðtted proÐle of the Ri (7) line of 13C14N (dotted line) for
comet Levy is compared to the observed spectrum (solid line) with the
blending of the adjacent P (18) line of the P-branch (1È1) taken into
account. The S/N of the Ri (8) line is 5.7.
FIG. 13.ÈSame as Fig. 10 but for comet Okazaki-Levy-Rudenko. The
S/N of the Ri(8) line is 4.5.
FIG. 14.ÈComparison of carbon isotope abundance ratios in nine
comets. CN ( Ðlled circles), HCN ( Ðlled triangles), C (open circles), the
mean carbon isotope ratio for all nine comets (solid 2horizontal line), the
solar system value (dotted line), and the local ISM value (dashed line) are
shown. The Ðgure indicates that comets have carbon isotope abundance
ratios consistent with the solar system value and possibly greater than the
present local ISM value.
Vol. 535
molecules in the cloud from which the protosun collapsed,
nor afterward in the protosolar nebula. Although the comet
sample for which the carbon isotope ratios have been
measured is small, the agreement among all comet and
solar system carbon isotope ratios indicates a minimum size
scale D100 AU for isotopic homogeneity in the protosolar
cloud.
Typically HCN and CN both represent a few tenths of a
percent of the volatiles in a comet nucleus (e.g., Wright et al.
1998 ; Eberhardt 2000). Simultaneous CN and HCN production rate measurements for comet Hale-Bopp (C/1995
O1) indicate that HCN can account entirely for the CN
observed (Jewitt et al. 1997 ; Wright et al. 1998 ; Ziurys et al.
1999). Moreover, the agreement between the HCN and CN
carbon isotope ratios in Hale-Bopp (Table 4) is also consistent with HCN being the parent of CN. Thus, it appears
that photodissociation of HCN can account adequately for
the observed CN gas.
Although most of the HCN originates directly from the
comet nuclei, signiÐcant fractions of CN and HCN have
been observed to arise from extended source regions in
comet comae. Roughly 20%È50% of the CN in comet
Halley (AÏHearn et al. 1986) and 20% of the HCN in HaleBopp (Wright et al. 1998) were estimated to have originated
in extended regions indicative of a dust source component
for these species. Large variations in the CHON grain
isotope ratios (12C/13C \ 1È5000) for the individual particles sampled by the Halley dust analyzer indicate that the
average dust carbon isotope ratio in Halley may have been
larger than the solar system value (Jessberger & Kissel
1991). However, the measured bulk CN carbon isotope
ratio including both the gas and dust components was
solar. The bulk HCN and CN isotope ratios for the other
comets listed in Table 4 are also weighted by the mix of the
nucleus and extended sources by di†erent degrees depending on the comet dust-to-gas ratios and the sizes of projected instrument apertures used to observe each comet. If the
large range in carbon isotope ratios measured for the dust
particles in comet Halley is typical of other comets and dust
does contribute signiÐcantly to the coma CN gases, then the
observed dispersion among our carbon isotope ratios
(Table 4) is remarkably small. Planned comet sample return
missions should provide important information on the
separate gas and dust carbon isotope ratios in comets.
The 14% di†erence between the solar system carbon
isotope abundance ratio (90 ^ 10) and the present local
ISM value (77 ^ 7) 8.5 kpc from the Galactic center may be
signiÐcant. If so, gradual 13C enrichment over the past 4.6
Gyr might explain the di†erence. Galactic chemical evolution models indicate that the primary source of 13C is mass
loss from red giants that have dredged up thermonuclear
processed material and gradually expelled it into the ISM
(e.g., Iben & Truran 1978 ; Timmes et al. 1995). In fact,
consistency to within a factor of 2 between the solar abundances, including the carbon isotope ratios, and a recent
Galactic chemical evolution model has been found (Timmes
et al. 1995). Additional evidence supporting a simple homogeneous Galactic chemical evolution can be found in the
observed decrease in the ISM carbon isotope abundance
ratio from the Galactic center out to the solar ring at 8.5
kpc (Wilson & Rood 1994). However, the sun has both a
high metal abundance, Z \ 0.02, and a high carbon isotope
ratio, leading some to suggest that the solar system abundances may not be representative of the Galactic ISM at the
No. 2, 2000
CARBON ISOTOPE ABUNDANCES IN COMETS
solar ring (8.5 kpc) 4.6 Gyr ago (e.g., Trimble 1991 ; Wilson
& Rood 1994 ; McAndrew 1997). In addition, variations in
the metal abundances among stars within the Orion association and the large dispersion in metal abundances among
halo dwarfs indicate chemical inhomogeneities on scales
smaller than star-forming regions (McAndrew 1997). Such
inhomogeneities may need to be incorporated into Galactic
chemical evolution models to account more accurately for
the solar system abundances.
6.1. Conclusions
Our results indicate the following. (1) The excitation
mechanism for the CN radical in the comae of comets is
well understood. (2) The CN carbon isotope abundance
999
ratios in Ðve comets are consistent with the solar system
value. (3) A lower limit, 12C14N/12C15N [ 200, found for
comet Levy is consistent with the solar system value. (4) The
C , HCN, and CN carbon isotope abundance ratios in nine
2
comets are all consistent with the solar system value, indicative that ion-molecule fractionation e†ects have not signiÐcantly altered the ratios after the molecules formed and
resided in cold interstellar clouds. (5) The 14% di†erence
between the present local ISM carbon isotope ratio and the
solar system value locked in 4.6 Gyr ago, if signiÐcant, may
indicate gradual 13C enrichment of the ISM. If so, the large
solar metal abundance may be difficult to explain by
current models of homogeneous Galactic chemical evolution.
REFERENCES
AÏHearn, M., Hoban, S., Birch, P. V., Bowers, C. Martin, R., & KlinglesKleine, M. L., Wycko†, S., Wehinger, P. A., & Peterson, B. A. 1995, ApJ,
mith, D. A. 1986, in 20th ESLAB Symp. on the Exploration of HalleyÏs
439, 1033
Comet, Vol. 1, ed. B. Battrick, E. J. Rolfe, & R. Reinhard (ESA SP-250 ;
Lecluse, C., Robert, F., Kaiser, R.-I., Roessler, K., Pillinger, C. T., & Javoy,
Paris : ESA), 483
M. 1998, A&A, 330, 1175
Amari, S., Hoppe, P., Zinner, E., & Lewis, R. S. 1993, Nature, 365, 806
Lis, D. C., et al. 1997, IAU Circ., 6566, 1
Anders, E., & Grevesse, N. 1989, Geochim. Cosmochim. Acta, 53, 210
McAndrew, A. 1997, ARA&A, 35, 503
Anders, E., & Zinner, E. 1993, Meteoritics, 28, 490
Prasad, C. V. V., Bernath, P. F., Frum, C., & Engleman, R., Jr. 1992, J. Mol.
ÈÈÈ. 1994, Icarus, 112, 303
Spectrosc., 151, 459
Clube, S. V. M., & Napier, W. M. 1984, MNRAS, 208, 575
Stapinski, T. E., Rodgers, A. W., & Ellis, M. J. 1981, PASP, 93, 242
ÈÈÈ. 1985, Icarus, 62, 384
Stawikowski, A., & Greenstein, J. L. 1964, ApJ, 140, 1280
Crane, P., Hegyi, D. J., & Lambert, D. L. 1991, ApJ, 378, 181
Swings, P. 1941, Lick Obs. Bull., 19, 131
Danks, A. C., Lambert, D. L., & Arpigny, C. 1974, ApJ, 194, 745
Timmes, F. X., Woosley, S. E., & Weaver, T. A. 1995, ApJS, 98, 617
Eberhardt, P. 2000, in Proc. 1996 COSPAR Colloq. Ser., Asteroids,
Trimble, V. 1991, A&A Rev., 29, 1
Comets, Meteors (Amsterdam : Elsevier), in press
Vanysek, V. 1977, in Comets, Asteroids and Meteorites, ed. A. D. DelHawkins, I., Craig, N., & Meyer, D. M. 1993, ApJ, 407, 185
semme (Toledo : Univ. Toledo Press), 499
Iben, I., & Truran, J. 1978, ApJ, 220, 980
ÈÈÈ. 1991, in Comets in the Post-Halley Era, Vol. 2, ed. R. Newburn,
Jaworski, W. A., & Tatum, J. B. 1991, ApJ, 377, 306
M. Neugebauer, & J. Rahe (Dordrecht : Kluwer), 879
Jessberger, E., & Kissel, J. 1991, in Comets in the Post-Halley Era, Vol. 2,
Wiedemann, G., Bjoraker, G. L., & Jennings, D. E. 1991, ApJ, 383, L29
ed. R. Newburn, M. Neugebauer, & J. Rahe (Dordrecht : Kluwer), 1075
Wilson, T. L., & Rood, R. T. 1994, ARA&A, 32, 191
Jewitt, D., Matthews, H. E., Owen, T., & Meier, R. 1997, Science, 278, 90
Wright, M. C. H., et al. 1998, AJ, 116, 3018
Kleine, M. L., Wycko†, S., Wehinger, P. A., & Peterson, B. A. 1994, ApJ,
Ziurys, L. M., Savage, C., Brewster, M. A., Apponi, A. J., Pesch, T. C., &
436, 885
Wycko†, S. 1999, ApJ, 527, L67